Drilling is a cutting operation in which material is removed from a workpiece to provide a bore in or through the workpiece. Drilling is carried out by advancing a rotating drilling tool or “drill” into the workpiece in the direction of the drill's longitudinal axis. Common drill configurations include, for example, twist drills and spade drills. A twist drill is characterized by one or more helical flutes disposed along at least a portion of the length of the drill and which terminate at a working end of the drill (the “drill tip”), which includes cutting edges. In contrast, a spade drill includes a wide cutting blade at the drill tip and lacks helical flutes along its length. Twist drills have a more complex geometrical design than spade drills due to the helical flutes, and this makes twist drills generally more difficult to manufacture.
Twist drills are manufactured as either non-composite twist drills or composite twist drills. A problem limiting the performance of non-composite twist drills is that the cutting speed (rotational speed of the cutting edge relative to the workpiece) varies from zero at the drill's center to a maximum cutting speed at the drill's periphery. Because of these variations in cutting speed, non-composite twist drills do not experience uniform wear along the cutting edge on the drill tip. The wear rate of a point on the cutting edge of the drill tip depends on the location of the point relative to the center of the cutting edge. The conditions promoting wear on a drill's tip can be significantly more aggressive at the periphery than at the center of the drill tip.
To address this problem, composite twist drills have been manufactured that include different materials or different material grades in different regions of the drill, allowing for different wear-resistance properties in the drill's central and peripheral regions. Such an arrangement has been adapted to optimize drilling performance. A composite twist drill has a monolithic construction but includes materials having different mechanical properties in different regions of the drill. However, it is relatively costly to manufacture composite twist drills because, for example, the production process involves additional steps related to pressing and sintering powdered precursors of the at least two different materials or material grades.
Accordingly, it would be advantageous to provide an improved twist drill design that addresses the non-uniform cutting edge wear experienced by non-composite twist drills, but that need not be manufactured using the relatively costly techniques used in making composite twist drills.